1. Field of the Invention
This invention relates to use of a chemical bath to process a substrate and, in particular, to extension of the useful life of such a chemical bath. Most particularly, the invention relates to methods for processing a substrate in which the substrate is rinsed after the substrate has been immersed in a chemical bath, and to methods for effecting such rinsing.
2. Related Art
Immersing a substrate in a chemical bath is a common step in the formation of some devices, such as, for example, semiconductor devices. In processing a semiconductor substrate (e.g., semiconductor wafer) to produce a semiconductor device, chemical immersion (and, in particular, wet chemical immersion in which the substrate is immersed in a liquid) is used for a number of different applications. For example, wet chemical immersion is used generally for many wafer cleaning, and bulk etch and strip applications. More particularly, one example of such wet chemical immersion is the use of sulfuric peroxide ("Piranha" cleaning) to remove organic material, such as photoresist, from the surface of a semiconductor wafer. Another example of such wet chemical immersion is the use of phosphoric acid to remove silicon nitride from the surface of a semiconductor wafer. Still another example of such wet chemical immersion is the use of a metal stripping solvent to remove metal from the surface of a semiconductor wafer.
After a substrate has been immersed in a chemical bath, the substrate must typically be cleaned to remove chemical residue from the substrate. Further, since, as indicated above, the chemical immersion is often used to remove material from the substrate or material previously formed on the substrate, it may be necessary to remove particles of the removed material that are redeposited on to the substrate during the chemical immersion. Particles may also be deposited on the substrate as the substrate is transferred from the chemical bath to the apparatus used for the subsequent cleaning process. In general, a cleaning process performed on a substrate subsequent to a chemical immersion of that substrate should desirably minimize the presence of any defects (e.g., contaminants or stains) that may have been introduced to the substrate as a result of the chemical immersion.
FIG. 1 is a flow chart illustrating the above-described two-step process (designated by the numeral 100), i.e., first immersing a substrate in a chemical bath (designated by the numeral 101), then cleaning the substrate to remove defects (designated by the numeral 102).
A typical way to accomplish the above-described cleaning (i.e., step 102 in FIG. 1) has been to "rinse" the substrate with a rinsing fluid. As will be apparent from the description below, "rinsing" can encompass a variety of operations in which a rinsing fluid is passed over the surface of the substrate with the purpose of removing defects present on the substrate. In semiconductor processing, deionized water is often used as the rinsing fluid; however, other fluids can be used.
In one conventional rinsing apparatus, the rinsing apparatus includes a rinsing tank and an overflow tank. Inlet pipes and (generally, any number can be used) are formed through the walls of the tanks and so that rinsing fluid can be flowed into the rinsing tank to immerse a substrate that is held in the tank by a support structure (not shown), such as a wafer boat. Sprayers and (again, generally, any number can be used) are positioned over the rinsing tank so that the sprayers and can spray rinsing fluid into the rinsing tank. A megasonic transducer is positioned near a bottom wall of the rinsing apparatus. The megasonic transducer can be excited (by well-known apparatus to impart megasonic vibrations to the rinsing fluid in the rinsing tank. A fluid-level sensor (the use of which is described further below) is positioned on a side wall of the rinsing tank. One or more doors (or other appropriate opening(s) or passage(s) that can be selectively opened and closed) are formed in a portion of the bottom wall that is part of the rinsing tank to enable rinsing fluid to be drained (preferably rapidly) from the rinsing tank.
In one previous method for rinsing a semiconductor substrate, termed "cascade rinsing," rinsing fluid is flowed into a rinsing tank, gradually filling the rinsing tank and immersing the substrate positioned therein, and eventually overflowing out of the rinsing tank. Thus, a steady flow of rinsing fluid past the substrate is established, the flowing rinsing fluid causing some defects to be removed from the substrate.
In another previous method for rinsing a semiconductor substrate, termed "dump rinsing," rinsing fluid is again flowed into a rinsing tank so that a substrate positioned therein is immersed. At some time after the substrate has been immersed by the rinsing fluid, the rinsing fluid in the rinsing tank is rapidly drained out of the rinsing tank. The rapid flow of rinsing fluid past the substrate effects removal of defects from the substrate and also causes contaminants collected at the bottom of the rinsing tank to be emptied from the rinsing tank.
In still another previous method for rinsing a semiconductor substrate, termed "spray rinsing," sprayers are used to spray rinsing fluid on to a substrate. Spraying rinsing fluid on to a substrate can directly remove defects from the substrate (e.g., dislodge particles on the substrate).
In still other methods for cleaning a semiconductor substrate, a megasonic transducer can be excited to impart megasonic vibrations to fluid that has been accumulated in a tank in which the substrate is at least partially immersed. As is known, the application of megasonic vibrations to a fluid in which a substrate is immersed facilitates the dislodgment of particles from the substrate, thereby enhancing the cleaning of the substrate.
A method for rinsing a semiconductor substrate in which the above-described techniques can be used has been implemented using the rinsing apparatus described above to. Such a method has been used to rinse a semiconductor substrate after immersion in a sulfuric peroxide bath.
In such a method, a rinsing tank is initially filled with rinsing fluid and a substrate is immersed in the rinsing fluid.
The rinsing fluid is dumped from the bottom of the rinsing tank after the substrate has been immersed for a desired length of time.
Rinsing fluid is flowed into the rinsing tank. The flow rate of the rinsing fluid into the rinsing tank is about 3.0 to 3.25 gallons per minute. Sprayers are also used to spray rinsing fluid on to the substrate. The flow rate of the rinsing fluid through the sprayers is about 0.5 to 0.75 gallons per minute. The sprayers continue spraying rinsing fluid on to the substrate until the final dump rinse, (described below) just prior to the end of the method.
A megasonic transducer is excited to produce megasonic vibrations in the rinsing tank when the level of the rinsing fluid in the rinsing tank has reached a first predetermined level. The first predetermined level can be established by the positioning and/or configuration of a fluid-level sensor: the sensor can be, for example, a device that senses when the fluid level in the rinsing tank has risen to the level of the sensor. The first predetermined level is a level at which approximately half of the substrate is immersed in the rinsing fluid.
The application of megasonic energy is stopped when the rinsing fluid has reached a second predetermined level. The second predetermined level can be, for example, when the rinsing tank is full.
Once the rinsing tank is filled, the rinsing fluid is dumped out of the rinsing tank. A brief period (e.g., several seconds or less) of "cascade rinsing" may occur prior to dumping the rinsing fluid from the rinsing tank. Such rinsing fluid overflow may occur incidentally or inadvertently, or because a brief period of overflow of the rinsing fluid is used as an easy way to ascertain when the rinsing tank is full. However, such "cascade rinsing" occurs for as short a period as possible (if at all), since such "cascade rinsing" is not intended as a rinsing mechanism, and because it is desirable to make the overall duration of the method as short as possible.
Typically, the steps of flowing rinsing fluid into the rinsing tank, exciting a megasonic transducer, stopping the application of megasonic energy, and dumping the rinsing fluid out of the rinsing tank are performed multiple times. For example, in one particular implementation of this method, those steps are performed four times. In that implementation, the method takes about 8 minutes to complete.
As discussed above, a substrate that has been processed by being immersed in a chemical bath is typically cleaned by rinsing, using, for example, one of the rinsing methods described above. Over time, the quality of a chemical bath will degrade because, for example, the bath becomes progressively more contaminated with, for example, particles of a material being removed from the substrate by the chemical bath, and because the composition of the chemical bath changes over time. As the quality of the chemical bath degrades, processed substrates will leave the bath with progressively more defects. Thus, it will generally become progressively more difficult for a particular rinsing method to adequately remove defects from the substrates processed in that chemical bath. At some point, the substrates leaving the bath are too contaminated for the rinsing method to clean adequately; at this point, the useful life of the chemical bath is over and the fluid of the chemical bath must be physically replaced or chemically treated before further processing with that chemical bath can continue. (Herein, "replacement" of a chemical bath fluid can refer to physical replacement of the fluid and/or chemical treatment of the fluid.)
The useful life of a chemical bath can be extended by recirculating the bath fluid through a filter system to remove particles from the fluid. (A chemical bath that is not so recirculated, or otherwise cleaned during use to remove particles from the bath, is often referred to as a "stagnant chemical bath," and that terminology is adopted herein. "Recirculated chemical bath" is used herein to refer broadly to any chemical bath which is cleaned during use to remove particles from the bath.) However, the chemical composition of the bath is not affected (or is affected only a small amount), so that, even in a recirculated bath, the bath fluid must eventually be replaced. In general, a recirculated chemical bath can used for somewhat less than twice as long as a stagnant chemical bath. While such extended bath life is clearly desirable, the extended bath life must be weighed against the increased cost--in, for example, equipment and manpower required for operation and maintenance--associated with the presence of the filtering system.
As can be appreciated, there are a number of costs associated with replacing the fluid of a chemical bath that make it desirable to replace the fluid as infrequently as possible so that those costs can be minimized. For example, throughput (the number of substrates processed) decreases as the frequency with which the fluid of a chemical bath must be replaced increases. If the fluid is physically replaced, the vessel containing the old fluid must be emptied and refilled. The vessel is also typically cleaned before being refilled. Additionally, chemical processing often takes places at elevated temperatures (e.g., about 155.degree. C. in a Piranha cleaning process to remove photoresist), necessitating that the old fluid be cooled before being replaced and the new fluid be heated before processing can begin again. Finally, it may be necessary to requalify operation of the chemical bath to ensure that the new fluid processes substrates in accordance with established specifications.
Frequent replacement of the fluid of a chemical bath increases material and labor costs as well. For example, the material cost (e.g., cost of water and chemicals) associated with the fluid used will obviously increase as the frequency with which the fluid of the bath is replaced increases. Frequent replacement of bath fluid can also increase wear and tear on the components of the chemical processing system: for example, the rapidity of deterioration in operation of thermocouples used to monitor the temperature of a heated chemical process increases as the thermocouples are thermally cycled more often, a circumstance attendant increased frequency of replacement of the bath fluid used in the process. Additionally, labor costs will increase with increased frequency of replacement of the fluid of a chemical bath, since, for any given duration of chemical processing, more fluid replacements--and thus more manhours--will generally be required to accomplish chemical processing for that duration of time.
Previously, little attention has been paid to precisely determining the useful life of a chemical bath, much less to determining how different rinsing methods affect the useful life of a chemical bath. One source has indicated that a sulfuric peroxide bath, when used with a rinsing method that is the same as, or similar to, the rinsing method described above, may be useful for up to 24 hours without occurrence of an unacceptable (e.g., greater than about 0.1 defects/cm.sup.2) increase in the number of defects present on the processed substrates after rinsing. (For a more detailed discussion, see "Post-Acid Rinse Enhancement through Megasonic Quickdump Rinsing," by Kai Chiu Wong, Michael B. Olesen and Greg Willits, published in Semiconductor FabTech, 6th edition, February 1997, pp. 825-831, the disclosure of which is incorporated by reference herein.) However, typically, the useful life of a chemical bath is arbitrarily established as a time period that provides a comfortable margin of assurance that the chemical bath will remain effective during the entire life of the bath. Such time periods are usually substantially less than 24 hours, e.g., 8 or 12 hours.
In view of the above, it is desirable to more precisely identify the useful life of a chemical bath used to process substrates and to use a particular chemical bath for as long as possible. It is also desirable to increase the useful life of a chemical bath beyond the time period for which such a chemical bath is currently used. More particularly, it is desirable to provide a rinsing method that more effectively removes defects from a substrate that has been immersed in a chemical bath than has heretofore been the case, so that a chemical bath used to process such substrates can be used for as long a time period as possible and, especially, can be used for a longer time period than such chemical bath has previously been used.